The cement industry does not fit the contemporary picture of a sustainable industry because it uses raw materials and energy that are non-renewable; extracts its raw materials by mining and manufactures a product that cannot be recycled. Through waste management and by utilizing the waste by-products from thermal power plants, fertiliser units and steel factories, energy used in the production can be considerably reduced. This cuts energy bills, raw material costs as well as green house gas emissions. In the process, it can turn abundantly available wastes, such as fly ash and slag into valuable products, such as geopolymeric concretes.
‘Geopolymer cement concretes’ (GPCC) are Inorganic polymer composites, which are prospective concretes with the potential to form a substantial element of an environmentally sustainable construction by replacing/supplementing the conventional concretes. GPCC have high strength, with good resistance to chloride penetration, acid attack, etc. These are commonly formed by alkali activation of industrial waste materials such as Fly Ash [FA] and Ground Granulated Blast Furnace Slag [GGBS], and have a very small Greenhouse footprint when compared to traditional concretes.
Recently, the incorporation of industrial and agricultural wastes in cement and concrete has gained considerable importance because of the requirement of environmental safety and more durable construction in the future.
Reference may be made to the article “Geopolymer Concrete—A Review” authored by M. I. Abdul Aleem and P. D. Arumairaj; International Journal of Engineering Sciences & Emerging Technologies, February 2012, Volume 1, Issue 2, pp: 118-122, wherein it is reported that in the conventional process of making geopolymeric materials, essentially three major constituents are required which are—(i) silicon and aluminium containing raw materials/wastes (ii) aqueous alkali hydroxide solutions like NaOH/KOH etc. (iii) sodium silicates/potassium silicates. The drawbacks of this process are: a) one of the reactants, namely sodium silicate (generally called as activator/activating solution in geopolymeric system) is viscous, which limits its dispersion and homogenization to desirable extent and b) as it essentially posses “inorganic frame work” only, its reactivity with other raw materials is restricted to considerable extent and c) it is costliest raw material in the externally added geopolymer reaction system.
Reference may be made to the article—Mechanism and Chemical Reaction of Fly Ash, Geopolymer Cement—A Review, authored by M. M. A. Abdullah, K. Hussin, M. Bnhussain, K. N. Ismail and W. M. W. Ibrahim; Int. J. Pure and Appl. Sci. Technol., 6 (1), (2011), pp. 35-44, wherein it is reported that the in-depth understanding of the chemical reactions taking place among the various reactants is necessary. The alkaline activator (sodium silicate) plays an important role as it is necessary for making geopolymeric materials. However, the drawbacks are that sodium silicate is always added externally in the conventional process of making geopolymers, which is a very costly chemical, and has inherent disadvantages as aforesaid.
Reference may be made to the article—The Effect of Sodium Silicate and Sodium Hydroxide on the Strength of Aggregates Made from Coal Fly Ash using the Geopolymerisation Method, Hamzah Fansuri, Didik Prasetyoko Zezhi′ Zhang and Dong-ke Zhang, Asia-Pac J. Chem. Eng., 7, 73-79, 2012, wherein it is reported that geopolymers have been made using fly ash and sodium hydroxide and sodium silicate solution. Further it is reported that the strength of geopolymers increases with increase in sodium hydroxide solution concentration, but the strength decreases beyond optimum concentration of sodium silicate solution. However, the process suffers from the drawback of requiring sodium silicate to be added externally.
Reference may be made to the article—The role of solid silicates on the formation of geopolymers derived from coal ash”. International Symposium of Research Students on Material Science and Engineering, Dec. 20-22, 2004, Chemai, India, page 1-13 wherein it is reported that the mechanism of geopolymerization clearly brings out that every reactant has its specific role. Further the activation potential of sodium silicate in comparison to other activators like potassium silicate and sodium hydroxide is relatively more and is based on the fact that sodium silicate already contains dissolved and partially polymerized sodium in the hydroxylated form as NaOH and silica in the form of ≡Si—OH. Both these species are in a very reactive form and therefore it reacts easily with siliceous and aluminous species of the raw materials e.g. fly ash and also get incorporated with them into the matrix. However, when sodium hydroxide alone is used without sodium silicate then the silicate and aluminate species from the raw materials are first dissolved by reaction with alkali and water leading to a relatively slow release of silicates and aluminates which are necessary to finally form the geopolymeric matrix. The drawbacks are that the process requires sodium silicate to be added externally.
Reference may be made to the article—Synthesis of pentacoordinate silicon complexes from SiO2, Richard M. Laine, Kay Youngdahl Blohowiak, Timothy R. Robinson, Martin L. Hoppe, Paola Nardi, Jeffrey Kampf & Jackie Uhm Nature 353, 642-644, 17 Oct. 1991, wherein synthesis of pentacoordinate silicon complexes are reported from silica gel, fused silica/s and, ethylene glycol and metal hydroxide. However, this process suffers from the drawback of using silica gel/fused silica and ethylene glycol which are costly and are added externally.
Reference may be made to the article—Penta- and hexacoordinate silicon mixed dichelates with the SiC2O2N(Cl) ligand environment, Irina Kalikhman, Boris Gostevskii, Vadim Pestunovich, Nikolaus Kocher, Dietmar Stalke and Daniel Kosta, Arkivoc 2006 (v) 63-77 wherein it is reported that hexa and penta coordinated complexes exhibit remarkable chemical flexibility and have been prepared from chlorine containing ligands. However, the drawbacks of the process are that it involves use of chlorinated compounds which are corrosive in nature.
Reference may be made to the article-Influence of C12A7 admixture on setting properties of fly ash geopolymer, Pavel Rovnaník, Ceramics—Silikáty 54 (4) 362-367 (2010) wherein fly ash based geopolymers were prepared using externally added sodium silicate as an alkali activator. The structure of geopolymer gel was mainly amorphous and the aluminum was present in hexacoordinated state. However, the drawback of the process is the addition of sodium silicate externally.
Reference may be made to the article—Understanding the relationship between geopolymer composition, microstructure and mechanical properties, Peter Duxson, John L Provis, Grant C Lukey, Seth W Mallicoat, Waltraud M Kriven, Jannie S J Van Deventer, Colloids and Surfaces A Physicochemical and Engineering Aspects (2005), Volume: 269, Issue: 1-3, Pages: 47-58 wherein it is reported that the Si/AI ratio plays an important role in determining the properties of the geopolymeric matrix. The properties depend on the microstructure of the matrix rather than simply on compositional characteristics. The geopolymer materials were prepared using alkaline activator solution possessing high concentration of soluble silicon which results in an increase in viscosity and limits its dispersion and chemical reactions with other raw materials and therefore the significant quantity of other raw material e.g. fly ash remains unreacted in the geopolymeric matrix and as a consequence this results in a decrease in the strength of the geopolymeric matrix. This process suffers from the drawback of limited dispersion and hindered chemical reactions of the activating solution and also its external addition.
Reference may be made to the article—Polymerization in sodium silicate solutions: a fundamental process in geopolymerization technology D. Dimas, I. Giannopoulou and D. Panias, J. Mater Sci. 2009, 44, 3719-3730, wherein it is reported that among the various raw materials used for making geopolymers the cost of sodium silicate solution determines the economic feasibility of the geopolymeric system. Additionally, the techno-economic feasibility of making geopolymers can be improved either by reducing the quantity of sodium silicate required or by developing novel compositions based on elements namely boron, Aluminum, Phosphorous along with transition metals namely iron, chromium and nickel etc. as these elements can provide gelatinous binder structures similar to that of sodium silicate. However, the drawbacks that this process suffers from are the limited dispersion and hindered chemical reactions of the activating solution and also it is added externally.
It is noted that the conventional process of making sodium silicate is highly energy intensive, as the sodium silicate is obtained by reacting silica containing compounds namely quartz/glass culets etc. with alkali carbonate and bicarbonate in the temperature range of 1100-1200° C. and at elevated pressure. However, this process generates considerable quantity of green house gas namely CO2 which is responsible for global warming problem and is energy intensive.
Reference may be made to the article—Nanocellulose from rice husk following alkaline treatment to remove silica. Leandro Ludueña; Diana Fasce; Vera Alvarez; Pablo Stefani, BIORESOURCES, 2011 vol. 6 (2) p. 1440-1453, wherein it is reported that multiple alkaline and acidic treatments of rice husk are required to obtain a variety of valuable products such as nano cellulose fibers, lignin, hemicelluloses and silica. However, the drawbacks that this process suffers from are the limited dispersion and hindered chemical reactions of the activating solution and also it needs to be added externally. Further, the process requires multiple treatments of the rice husk for obtaining various valuable materials.
Reference may be made to U.S. Pat. No. 4,964,995 wherein it is reported that, the application of kraft processes helps in extracting the lignocellulosic material from various fibers/materials using sodium hydroxide/sulfide solutions. However, the drawbacks that this process suffers from are the limited dispersion and hindered chemical reactions of the activating solution and also it needs to be added externally. Further, the process utilizes sulphide containing chemicals which are harmful in developing geopolymeric materials.
Further it is pertinent here to mention that use of rice husk ash as silica source for making geopolymeric materials has been widely reported by several workers. On the contrary, very limited use of “rice husk as such” and that to as mere reinforced material in geopolymeric matrix has been mentioned in the literature.
Reference may be made to the article—Silica Derived from Burned Rice Hulls, M. F. de Souza, W. L. E. Magalhaes, M. C. Persegil, Material Research, vol. 5, No. 4, 467-474, 2002, wherein it is reported that silica has been obtained from rice hulls by burning at elevated temperatures. The drawbacks that this process suffers from are the limited dispersion and hindered chemical reactions of the activating solution and that it needs to be added externally. Further, it is an energy intensive process and liberates green house gas namely CO2.
Reference may be made to US 20110271876, wherein it is reported that geopolymeric materials were prepared from rice husk ash, blast furnace slag, bauxite, alumina slag or tailings, or powdered alumina oxide, dry sodium silicate or sodium silicate premixed with water or dry potassium silicate or potassium silicate mixed with water. The drawbacks that this process suffers from are the limited dispersion and hindered chemical reactions of the activating solution and that it needs to be added externally. Moreover, the process involves burning of rice husk to obtain rice husk ash which consumes energy and generates CO2.
Reference may be made to the article—A novel mesoporous lignin/silica hybrid from rice husk produced by a sol-gel method Yuning Qu, Yumei Tian, Bo Zou, Jian Zhang, Yunhui Zheng, Lili Wang, Ying Li, Chunguang Rong, Zichen Wang, Bioresource Technology, Volume 101, Issue 21, November 2010, Pages 8402-8405, wherein reported are mesoporous lignin/silica hybrid prepared from rice husks using H2SO4 which separates lignin and silica and results in the formation of composite of lignin in silica powder. Nevertheless, the drawbacks are that this process suffers from the limited dispersion and hindered chemical reactions of the activating solution and also it needs to be added externally. Further, the lignin/silica hybrid material contains silica powder separately and the use of sulfuric acid dehydrates the organic species.
Reference may be made to U.S. Pat. No. 5,244,726, wherein a foamed composite is prepared by using alkali metal silicate-based activator, flyash, sodium laurel sulfate which contains dispersed inorganic particulates, organic particulates, or mixed inorganic and organic particulates from the group consisting of expanded polystyrene beads and polyethylene tetra phthalate polyester chopped fibers. However, the drawbacks that this process suffers from are the external addition and limited dispersion and hindered chemical reactions of the activating solution. In addition, the process uses alkali metal silicate-based activator (sodium silicate), polystyrene beads and polyethylene terephthalate polyester chopped fibers possessing very less compatibility with geopolymeric matrix. It is also reported that the process uses rice husk as a mere dispersed phase component to utilize its reinforcing characteristics only.
Reference may be made to the article—Fly Ash Porous Material using Geopolymerization Process for High Temperature Exposure, Mohd Mustafa Al Bakri Abdullah, Liyana Jamaludin, Kamarudin Hussin, Mohamed Bnhussain, Che Mohd Ruzaidi Ghazali and Mohd Izzat Ahmad, Int. J. Mol. Sci. 2012, 13, 4388-4395, wherein porous geopolymeric material has been prepared from fly ash, alkali activator, sodium hydroxide, aluminium powder and hydrogen peroxide. It was observed that the strength of the geopolymeric matrix increases as a result of heat treatment. However, the drawbacks that this process suffers from are the external addition and limited dispersion and hindered chemical reactions of the activating solution.
Reference may be made to the article—Mechanical and microstructural properties of alkali-activated fly ash geopolymers, by M Komljenović, Z Bascarević, V Bradić, Journal of Hazardous Materials (2010), Volume: 181, Issue: 1-3, Publisher: Elsevier, Pages: 35-42, wherein properties of geopolymer obtained by alkali-activation of fly ash (FA), using aqueous solutions of Ca(OH)(2), NaOH, NaOH+Na(2)CO(3), KOH and sodium silicate (water glass) of various concentrations as alkali activators have been described. The drawbacks are the external addition as well as limited dispersion and hindered chemical reactions of the activating solution.
Reference may be made to the article—Alkali Activated Fly Ash: Effect of Admixtures on Paste Rheology, M Criado, A Palomo, Ana Fernandez-Jiménez, P F G Banfill, Rheologica Acta (2009) Volume: 48, Issue: 4, Pages: 447-455, wherein recited is the conventional practice of utilization of organic species containing chemical admixture as water reducing agent for cement-concrete system. The process aims to achieve primarily, reduction in the water cement ratio to improve mechanical strength and secondarily to increase workability, reduce segregation, decrease permeability etc. Further, the salts of lignosulphonic acid or lignin provide excellent water reduction and produce good strength characteristics in conventional cementitious concrete systems. The conventional admixtures are used as external additives in minor amounts to improve the characteristics of cement-concrete matrix. The basic action of this admixture is to reduce water requirement in cementitious materials. However, it is important here to note that the characteristic of admixtures is not useful for geopolymeric systems as water is liberated during geopolymerisation and it does not require water for curing. Moreover, the disadvantages of this process are the limited dispersion and hindered chemical reactions of the externally added activating solution and that these organic containing admixtures are not workable in geopolymeric systems due to high alkaline environment.
Reference may be made to US 20110284223, wherein it is reported that organic compounds containing at least one nitrogen atoms such as lignin amine have been used as chemical admixtures and are suitable as a retarder in geopolymeric compositions. However, the drawbacks are that the activating solution needs to be added externally coupled to its limited dispersion and hindered chemical reactions. Further, the organic compounds containing nitrogen atoms are used externally and as mere retarders in geopolymeric systems which helps only in improving their setting time and workability etc. These compounds are not at all essentially a part of the basic reaction mixture required for making conventional geopolymeric materials.
From the hitherto reported prior art and based on the drawbacks of the conventional processes, the various issues that need to be addressed and problems to be solved are summarized here as under. The last two decades have witnessed significant achievements in the area of making advanced multifunctional geopolymeric materials and one of them is the development of inorganic geopolymeric materials. However, the development of this unique technology suffers from the following drawbacks:
1. Firstly, the work so far carried out in the area of development of geopolymeric materials is basically restricted and is essentially based on “inorganic polymeric materials” only.
2. Secondly, the most important and primary reactant required for making geopolymeric materials namely sodium silicate (activator/activating solution), possesses only “inorganic frame work” which sterically hinders its chemical reactivity with various constituents of the other raw materials. Further, when added externally, its limited dispersion and chemical reaction leads to the segregation of various phases in the matrix and thus results in a non-homogeneous matrix exhibiting poor geopolymeric characteristics.
3. The functionality of the conventional geopolymeric materials is limited as basically they are an Inorganic matrix.
4. The sodium silicate used in making the conventional geopolymeric materials is the costliest constituent among all the raw materials necessary for making geopolymeric materials; as the conventional process of making sodium silicate is highly energy intensive because the sodium silicate is obtained by reacting silica containing compound namely quartz/glass culets etc. with alkali carbonate and bicarbonate in the temperature range of 1100-1200° C. at elevated pressure. Another drawback of the process of making sodium silicate is the generation of green house gas namely CO2 responsible for global warming problem.
5. The silica derived from burning of rice husk has been utilized in some of the literature reports for making geopolymers. However during the burning process, the amorphous silica is substantially converted to less reactive crystalline phase and is associated with un-burnt carbon as impurity. Further, during the burning process CO2 is generated.
6. Some of the conventional processes of making advanced geopolymeric composites use rice husk as a mere dispersed phase component to utilize its reinforcing characteristics.